I've seen on recipe sites that it calculates an BU:GU ratio for your recipes. Could someone explain what this is?
It's a rough guide to how much bitterness vs sweetness is in the beer.
BU stands for bittering units, most often as IBU - international bittering unit.
GU stands for gravity unit - the number in thousandths after the 1 of the original gravity. A 1.040 beer has 40 GUs, while a 1.105 beer, has 105 GUs. It's a rough guide to how much sugar and thus sweetness is in the beer.
The BU:GU ratio is the IBUs divided by the gravity units. It represents the amount of bitterness balanced with the sweetness. Higher values mean more bitterness. The scale is roughly 0.25-0.35 for wheats, 0.4-0.8 for the majority of ales, and 1.0+ for IPAs.
For example, if you brewed an IPA with an OG 1.060 and hopped with 60 IBUs of hops, then the BU:GU ratio would be 1.0. Whereas a wheat at 1.045 OG with 15 IBUs of hops, would have a BU:GU ratio of 0.33.
The suggested BU:GU figures aren't hard and fast, but serve as a guideline. For example, some of my English bitters have approached 1.0 for BU:GU ratio simply because they are low in gravity. The numbers are just a guideline to get in the ballpark for the style.
It's the ratio of Bittering Units to Gravity Units. Higher numbers indicate more perceived bitterness. The intention is to create a balanced beer. The high finishing gravity of strong beers offsets the perceived bitterness. Hence, strong beers need more hops to achieve the same level of perceived bitterness as weaker beers.
You calculate the BU:GU by taking the total IBUs for the recipe, and dividing by the gravity points. Gravity points are calculated as: (1 - SG) * 1000.
For example, suppose you're brewing an American Pale Ale with a starting gravity of 1.055 and 40 IBUs:
BU:GU = 40 / ((1 - 1.055) * 1000) = 40 / 55 ~= 0.73
I don't know if there's a definitive guide to BU:GU ratios for various styles, but the Mad Alchemist has a nice tool that provides some guidance.